Selenoprotein A, a component of glycine reductase complex from Clostridium sticklandii, has been reported to be a glycoprotein by the PAS-staining method. Since the function and biosynthetic mechanism of oligosaccharides linked to bacterial glycoproteins has not been elucidated thus far, we started analyzing the oligosaccharide linked to selenoprotein A. Carbohydrate analyses by two methods revealed the absence of sugars commonly found in eucaryotic glycoproteins (glucose, mannose, galactose, xylose, fucose, rhamnose, N-acetylglucosamine, N- acetylgalactosamine) in this enzyme. This result suggested that selenoprotein A might not be a glycoprotein. Although the molecular weight of this enzyme was calculated to be 17,142 from the deduced amino acid sequence, laser desorption mass spectrometry (LDMS) showed the molecular mass of this enzyme is 18,049. This fact suggested that selenoprotein A is modified by an unidentified compound(s) with a molecular weight of nearly 1,000. To determine which amino acid is modified by this unknown compound, the amino acid sequence of seleno- protein A was analyzed. Eighty-five percent of the total amino acid sequence has been revealed by Edman degradation and LDMS analyses of purified peptides. However, no modified amino acid has been identified so far. LDMS analysis of selenoprotein A oxidized by periodic acid indicated that this oxidation caused a peptide bonding cleavage and produced about 14 Kda and about 4 Kda peptides. The 4 Kda peptide was modified by 2,4- dinitrophenylhydrazine indicating the presence of an aldehyde or ketone group produced by periodate oxidation.